Silicon crystallization is a step that is often used in the manufacture of thin-film transistor (TFT) active-matrix LCDs, and organic LED (AMOLED) displays. The crystalline silicon forms a semiconductor base, in which electronic circuits of the display are formed by conventional lithographic processes. Commonly, crystallization is performed using a pulsed laser beam shaped in a long line having a uniform intensity profile along the length direction (long-axis), and also having a uniform or “top-hat” intensity profile in the width direction (short-axis). In this process, a thin layer of amorphous silicon on a glass substrate is repeatedly melted by pulses of laser radiation while the substrate (and the silicon layer thereon) is translated relative to a delivery source of the laser-radiation pulses. Melting and re-solidification (recrystallization) through the repeated pulses, at a certain optimum energy density (OED), take place until a desired crystalline microstructure is obtained in the film.
Optical elements are used to form the laser pulses into a line of radiation, and crystallization occurs in a strip having the width of the line of radiation. Every attempt is made to keep the intensity of the radiation pulses highly uniform along the line. This is necessary to keep crystalline microstructure uniform along the strip. A favored source of the optical pulses is an excimer laser, which delivers pulses having a wavelength in the ultraviolet region of the electromagnetic spectrum. The above described crystallization process, using excimer-laser pulses, is usually referred to as excimer-laser annealing (ELA). The process is a delicate one, and the error margin for OED can be a few percent or even as small as ±0.5%
There are two modes of ELA. In one mode, the translation speed of a panel relative to the laser beam is sufficiently slow that the “top-hat portion” of the beam-width overlaps by as much as 95% from one pulse to the next so any infinitesimal area receives a total of about 20 pulses. In another mode referred to as advanced ELA or AELA the translation speed is much faster and in a single pass over a panel the irradiated “lines” have minimal overlap and may even leave un-crystallized space therebetween. Multiple passes are made such that the entire panel is irradiated with a total number of pulses that may be less than in an ELA process to produce equivalent material.
Whichever ELA mode is employed, evaluation of crystallized films on panels in a production line is presently done off line, by visual inspection. This inspection is entirely subjective and relies on highly trained experienced inspectors, who through their experience are able to correlate observed features of the panels with very small changes, for example less than 1%, in energy density in the crystallizing beam. In a manufacturing environment, the process of visual analysis and establishing if a change of process energy density is necessary typically takes between about one and one and one-half hours from when the crystallization was performed, with a corresponding adverse effect on production line throughput of acceptable panels.
There is a need for an objective method of evaluation of the ELA process. Preferably, the method should be capable at least of being implemented on a production line. More preferably, the method should be capable of being used for quasi real-time evaluation in a feedback loop for automatically adjusting process energy density responsive to data provided by the evaluation.